Signage

The start of a speed limit is usually marked with a speed limit traffic sign. Speed limit signs can appear near political borders and road intersections, and in some cases speed limit reminder signs appear at regular intervals. Political borders can range from country borders to city limits.

Occasionally different units of speed measurement are used on each side of a border. For example, Northern Ireland (part of the UK) still uses miles-per-hour (MPH) for speed limits and miles for distance, while the Republic of Ireland uses the standard international system (SI) of kilometres per hour (km/h) for speed limits and kilometres for distance. The Republic of Ireland completed the changeover from imperial units to SI units in early 2005. The UK and the US are the only major nations still using the imperial units system. The US has no intentions to convert to SI units in the foreseeable future, and in fact, reverted to its current imperial units in states that had both imperial and SI systems such as California and Arizona. However, Ohio still has some signs listed with SI distances and speeds on its exit distance and speed limit signs (such as 70 mph / 110 km/h, or 3 miles / 5 km to next exit).

Factors in Setting Speed Limits

Speed limits are set based on many factors, such as road features, crash records, legal statutues, administrative judgement and engineering judgement. Two common measures for setting speed limits are the design speed of the road, and the eighty-fifth percentile of travel speeds.

Note that highway design practices in other countries (Australia, Canada, France, Germany, Great Britain, and Switzerland) were surveyed. At one time, most of the countries’ policies on design speed were identical to current U.S. policy and are still similar to U.S. practice.

Design speed

In the United States, the design speed is officially defined as "a selected speed used to determine the various geometric design features of the roadway" according to the 2001 AASHTO highway design manual, commonly referred to as the "Green Book". Previous versions of the Green Book referred to design speed as the "maximum safe speed that can be maintained over a specific section of highway when conditions are so favorable that the design features of the highway govern"; however the 2001 edition removed the term “safe” in order to avoid the misperception that speeds greater than the design speed were “unsafe.”

Safe observed operating speeds can exceed the nominal design speed because (a) design speed specifies roadway's most restrictive feature (e.g., a curve, bottleneck, hill, etc.) rather than representative features along a roadway section and (b) actual roadway design may exceed the minimum design specifications. On busy roads, capacity and congestion are primary limiting factors on speeds.

Design speed is therefore considered only a "first guess" at an appropriate speed limit.

85th percentile rule

In the United States, traffic engineers rely on the 85th percentile speed to establish speed limits. They note that traffic laws that reflect the behavior of the majority of motorists are found to be successful, while laws that arbitrarily restrict the majority of motorists encourage violations, lack public support and usually fail to bring about desirable changes in driving behavior.

The idea is that the speed limit should be set to the speed that separates the bottom 85% of vehicle speeds from the top 15%. The 85th percentile closely corresponds to one standard deviation above the mean of a normal distribution.

Traffic engineers observe that the majority of drivers drive in a safe and reasonable manner as demonstrated by consistently favorable driving records. The 85th percentile speed is how drivers "vote with their feet." Studies have shown crash rates are lowest at around the 85th percentile. Vehicles traveling over the 85th percentile speed (or faster than the flow of traffic) have a higher crash risk than vehicles traveling around or modestly below the 85th percentile speed.

Most U.S. jurisdications report using the 85th percentile speed as the basis for their speed limits, so the 85th percentile speed and speed limits should be closely matched. However, a review of available speed studies demonstrates that the posted speed limit is almost always set below the 85th percentile speed by as much as 8 to 12 mph.

The Northern Territory signals the end of its built-up area default, OR END of a posted speed restriction sign on a length of road leading to a rural environment - by use of the speed-derestriction' sign (//). This is an international road traffic sign held in all "United Nations Conventions on Road Traffic, Road Signs and Signals", where it is catalogued as a C,17a - meaning "End of all local prohibitions imposed on moving vehicles". That said, certain license holders, such as learner drivers are restricted in speed by 'license conditions'. Heavy vehicles are also speed restricted by way of separate vehicle construction and other legal regulations. It should be said NT police do NOT tolerate dangerous speeds or behaviour.

Also used to signal the end of a posted speed limit - leading to a rural area default speed limit - is the END speed-limit sign. This is a unique Australian-designed sign which contains the word "END" and a number in a circle beneath this which represents the ceasing speed limit. It is typically used where the road beyond has certain hazards such as hidden driveways, poor camber, soft edges and other hazards where the road authority feels a posted speed limit sign might be too dangerous or otherwise unwarranted. It is intended to invoke particular caution. Again, the rural default applies as a maximum.

Speed traps are used in almost all areas of the country including NT above. Tolerance is from 8% to 10% in most states but only 3 km/h in Victoria, an issue which has caused a lot of controversy in that state. Measures used are police radar, fixed speed cameras, unmarked stationary cameras, fixed 'point to point' cameras and laser.

Historical limits

Prior to metrication in the 1970s the rural default urban speed limit was 35 mph (56km/h)and the rural default speed limit speed limit (if it existed) was 65 mph (105km/h). At metrication all limits and advisory signs were converted to the nearest 10km/h so the urban limit became 60km/h and the rural limit 110km/h, a slight increase for both. These limits were altered around 2000 to their current values as part of the introduction of uniform national road rules between the states.

Note that where more than one limit is given per road, it usually indicates a difference between provinces; however, within provinces, different roads of the same classification have different speed limits. For example, in Alberta and Nova Scotia some freeways have a limit of 100 km/h, while others have a speed limit of 110 km/h (70 mph). In Ontario, all freeways have a maximum speed limit of 100 km/h unless there is a lower posted limit. Speed limits are generally lower in Ontario and Quebec on comparable roads than in other Canadian provinces, except perhaps British Columbia. Examples of this disparity include rural two-lane highways in Ontario which have a standard speed limit of 80 km/h, while comparable roads in other provinces have standard speed limits of 90–100 km/h.

In British Columbia, a review of speed limits conducted in 2002 and 2003 for the Ministry of Transportation found that posted limits on investigated roads were unrealistically low for 1309 km and unrealisticly high for 208 km. The reports recommended to increase speed limits for multilane limited access highways constructed to high design standards from 110 km/h to 120 km/h. MoT Speed Review Report As described in that report, the Ministry is currently using "...Technical Circular T-10/00 [...] to assess speed limits. The practice considers the 85th percentile speed (the speed at or below which 85% of the motorists are traveling), road geometry, roadside development, and crash history."

In Canada, as in most other locales, speed violation fines are double (or more) in construction zones.

Semi-expressways and city express routes (called kuaisu gonglu in Chinese, meaning "high speed public road") generally have lower speed limits topping out at around 100 km/h , and in some cases, the speed limit may be lower.

On China National Highways (which are not expressways), a common speed limit is 80 km/h . In localities, speed limits may drop to 40 km/h .

In reality, few people drive according to the speed limits, and on most roads, enforcement cameras are non-existent.

On some designated "fast through routes" in cities, speed limits can go all the way up to 80 km/h . Otherwise, speed limits remain 70 km/h on roads with two uninterrupted yellow lines and 60 km/h or even 50 km/h otherwise. Signage in towns and on expressways is often present.

Minimum speed limits on expressways vary. A general minimum speed limit of 60 km/h is in force at all times (although traffic jams more than thwart it). The maximum speed limit, as posted on Chinese motorways is 120 km/h. This is a recent change. China is teaching EU/HK/GB like 'lane discipline' to its driver candidates.

*Motor routes: Roads with two or more lanes (dual carriageway), a median, and a minimum speed of 60 km/h (40 mph).

Remarks:
1 130 (80) is the recommended maximum speed on motorways as indicated by a blue sign. Many sections of the German motorway network are now also covered by enforcable speed limits, usually ranging from 80 to 120 km/h depending on local conditions. It is usual for drivers involved in crashes who were exceeding the 'recommended' speed limit to be held to be at least partly at fault, regardless of the circumstances of the crash.
² For motorcycles 110 (70).
³ Two-lane expressways: 130 (80); three-lane expressway: 150 (95) (since 2003, the speed limit of 150 km/h (95 mph) is only valid when signed).
4 Automobile with weighty trailer: 80 (50); Truck with weighty trailer: 70 (45).
5 Automobile with weighty trailer: 100 (65); Truck with weighty trailer: 80 (50).
6 During the winter months, when conditions are often bad, all Finnish motorways have a speed limit of 100 km/h (65 mph) or less.
7 Need to be licensed from the German Technical Inspection Authority (TÜV).
8 Effective January 20, 20059 A provisional increase of the speed limit on motorways from 90 to 100 km/h was made permanent when it turned out the number of accidents decreased.
10 Signs are posted in miles per hour, a situation unlikely to change in the near future.
11 100km/h is default limit on all National Routes regardless of design standard.
12 Iceland doesn't have expressways/motorways in the traditional sense. There is really only one such road, with three and four lanes and no traffic lights. It is within city limits, and the maximum speed is 80 (50). You are not pulled over if you keep below 100 (60). A general rule of thumb in Iceland is if you exceed the speed limit by no more than 20 (10) you are not likely to be pulled over and fined. In some parts of the country this is closer to 10 (5).
13 Between 90 and 110 km/h depending on how many lanes the road has.

In most European states there is a general speed limit of 50 km/h (30 mph) inside towns.

Comments

The first British motorways did not have imposed speed limits. However, after a series of severe crashes a temporary speed limit of 70 miles per hour (110 km/h) was enforced in 1965, which was made permanent in 1967. It was reduced to 50 mph (80 km/h) in response to the 1973 oil crisis and restored to 70 mph (110 km/h) in 1974. The Association of British Drivers have called for the limit to be increased. The opposition Conservative Party are now proposing to raise the limit to 80 miles per hour (130 km/h) where appropriate.

On French autoroutes, there is a de facto variable speed limit. In dry weather an autoroute has a speed limit of 130 km/h (80 mph), when raining the speed limit is reduced to 110 km/h (70 mph). In 2005, a governmental report advised lowering this speed to 115 km/h in order to save fuel and reduce accident risks, but this proposal was badly received. Since 2002, the French government has installed a number of automatic radar guns on autoroutes, routes nationales and other major thoroughfare, in addition to radar manned by the Police or Gendarmerie. The French authorities have credited this increase in traffic enforcement for a 21% drop in road fatalities from 2002 to 2003.

The German Autobahnen are famous for having no speed limits for cars over much of their length. Blanket speed limits do apply for trucks, buses and cars pulling trailers. Speeds over 200 km/h (125 mph) are not uncommon, but there is a recommended speed (Richtgeschwindigkeit) of 130 km/h (80 mph). In case of a crash, insurance payments can be dropped where the recommended speed is exceeded. Some areas have compulsory speed limits to reduce the noise or for safety reasons. Many car manufacturers (including Mercedes, BMW and Audi) limit the speed of their cars electronically to 250 km/h (155 mph) although this is not a legal requirement.

Swiss Autobahnen are limited to 120 km/h (75 mph) as a maximum speed limit. Semi-motorways, known as "motor roads" or Autostrassen, have a generally lower speed limit of 100 km/h (65 mph).

For a period about 1990 to 1995, Sweden banned the highest limit 110 km/h (70 mph) in the large-city provinces, citing environmental reasons. 90 km/h (55 mph) limits were introduced on most motorways, the lowest in Europe. The term "large-city province" was defined as any province having one of the three large cities with suburbs. That meant that the west coast motorway E6 had 90 km/h all along its (then) about 250 km of motorway, but some ordinary roads in less densely populated provinces had 110 km/h. This ban was later removed because the limit was neither popular nor much obeyed.

India

India has a speed limit in towns and this is usually signed. It is a contracting party to the "United Nations Conventions on Road Traffic, Road Signs and Signals". Road condition is historically poor, discouraging high speeds, but in Andhra Pradesh and Tamil Nadu in particular, the roads have been substantially improved in recent years due a buoyant economy.

India for the past few years has embarked on a large road building effort with some 47,000 kilometres of world class motorway reportedly under construction, and finalised. (National Highways Agency India). On highways where speed restrictions end, it so signaled by use of the 'speed derestriction' sign, known by catalogue number in the UN Convention as a C,17a 'End of all local prohibitions imposed on moving vehicles'. (Sign, third-row from the bottom).

Malaysia

The speed limits in Malaysia is 110km/h on closed toll expressway. On federal,state and muncipal roads speed limit are from 50 to 90km/h but usually the default speed limit is 90 km/h and the speed limit is reduced to 60 km/h in urban areas. The highway police have monitored their speed limit as well below on bridge and signboards.

New Zealand

Speed limits in New Zealand range from 20 km/h to 100 km/h. Specifically:

20 km/h (10 mph) past school buses and accident sites

30 km/h (20 mph) past roadworks

50 km/h (30 mph) in most urban areas

60 km/h (40 mph) for many city arterial routes

70 km/h or 80 km/h (45–50 mph) on highways through built-up areas, or on dangerous or older roads.

100 km/h (65 mph) on expressways and highways

(It must be noted that New Zealand misuse, like GB:- the international speed de-restriction sign (//) to signal a maximum speed limit of 100km/h. Tourists to New Zealand should remember this when visiting. "End of all local prohibitions on moving vehicles" as held in international law in the governing Convention is hardly 100km/h, so be warned).

In general, speed limits are reactionary figures to an observed average volume and its potential for flucuation. In other words, an urban interstate with many interchanges/junctions near each other will have a significantly lower speed limit then a rural highway that sees relatively little traffic and have several miles between two interchanges. Due to this, the more urbanized east has lower speed limits on average then the more spread out west.

Speeding

Speeding is defined by the U.S. Federal Government as either exceeding posted limits or driving too fast for conditions [1]. Speeds in excess of posted maximum speed limits account for most speed-related traffic citations. Most speed-related crashes involve speed too fast for conditions [2] such as limited visibility or reduced road traction. Variable speed limits offer some potential to reduce speed-related crashes, but due to the high cost of implementation exist primarily on motorways, while most speed-related crashes occur on local and collector roads [3] Speed-related crashes can also occur at speeds below 30 miles per hour; for example, truck rollovers on exit ramps.

Enforcement

Prior to the invention of radar, speed limits were normally enforced by clocking vehicles travelling through speed traps. Clocking a vehicle simply means timing how long it takes for the automobile to pass between two fixed landmarks along a roadway, from which the vehicle's average speed could easily be determined. Setting up a speed trap that could provide legally satisfactory evidence was usually time consuming, however, and early speed traps were often difficult to hide. As a result, organizations such as the Automobile Association could often keep fairly accurate records of speed trap locations.

In the early 21st century, police used radar, LIDAR, planes, and automated devices. Officers may also use a method called pacing: following a car for a certain time to establish speed using the calibrated speedometer of the patrol car. Recently, Automatic Number Plate Recognition (ANPR) camera systems have been used which time a vehicle between long sections of road (approximately one mile), calculating the average speed between two points. This method eliminates the risk of heavy braking at the locations of conventional speed cameras, but may raise privacy issues.

In several countries, notably the Netherlands and the United Kingdom, an increase in automated speed enforcement has resulted in a significant increase in the number of fake number plates. In France, the use of automated enforcement has been credited with contributing to a substantial reduction in fatalities.Template:Citation needed Most Western European countries now use automated enforcement on at least some roads.

Speed limit policy can affect enforcement. According to the AASHTO, "experience has ... shown that speed limits set arbitrarily below the reasonable and prudent speed perceived by the public are difficult to enforce, produce noncompliance, encourage disrespect for the law, create unnecessary antagonism toward law enforcement officers, and divert traffic to lesser routes[.]"[4] Arbitrarily low limits can turn otherwise reasonable drivers into habitual speed limit violators.

Safety and efficacy

Essential physics

Forces in a motor vehicle collision are proportional to the square of the speed change (sometimes referred to as "delta-V", symbolized as δv) at impact. This means that crash forces rise much faster than speed. The probability of a fatality is proportional to the fourth power of the speed change at impact [5], rising much faster than crash forces.

To illustrate these statistics, suppose two vehicles crash into a massive, fixed object, and one vehicle’s speed is 10% greater than the other vehicle. The faster vehicle will experience 21% higher crash forces, and its occupants will experience a 46% higher probability of a fatality.

When interpreting this, it should be noted that crashes with dramatic, sudden speed changes that terminate almost all velocity are atypical. These kinds of atypical crashes can include head on collisions or collisions with massive, fixed objects like trees or concrete bridge piers.

Although the basic relationship between vehicle speed and crash severity is unequivocal and based on the laws of physics, the probability of a crash as well as crash severity can be mitigated. Safety devices like crash attenuators, barriers, or wide medians are examples. The highest degree of mitigation is found on motorways (which may be called freeways, limited access highways, also Autobahns, Interstates or other national names), which are internationally documented as being the safest roads per-mile-travelled despite their higher speeds, due to designing out of most conflict opportunities as well as restricted access.

Speed limits, actual speeds, and aggregate safety

The 1998 Synthesis of Safety Research Related to Speed and Speed Management sponsored by the US Federal Highway administration found, "on freeways and other high-speed roads, speed limit increases generally lead to higher speeds and crashes." Increasing a speed limit by 4 mi/h would increase the average speed by 1 mi/h and increase injury accidents by 5%. The report cautions that "changing speed limits on low and moderate speed roads appears to have little or no effect on speed and thus little or no effect on crashes". The report noted that traffic calming significantly reduced speeds and injuries in treated areas but that the decrease may be due to reduced traffic volumes. The report also suggests that "variable speed limits that adjust with traffic and environmental conditions could provide potential benefits" as most of the speed related crashes involve speed too fast for conditions.

The report noted the landmark study (D. Solomon, "Accidents on Main Rural Highways Related to Speed, Driver, and Vehicle", Federal Highway Administration, Washington, DC, July 1964) that observed a "U-shaped curve" of crash probability versus speed, where crash rates were lowest for travel speeds near the mean speed of traffic, and increased with greater deviations above and below the mean. Subsequent research has found that "The occurrence of a large number of crashes involving turning maneuver partly explains the increased risk for motorists traveling slower than average and confirms the importance of safety programs involving turn lanes, access control, grade separation, and other measures to reduce conflicts resulting from large differences in travel speeds."

Speed and crash factors

Some safety factors are not always under the full control of the driver, such as driver alertness and distractions, road conditions, weather, daylight availability, actions and alertness of other drivers, and wildlife. While these factors are not directly related to vehicle speed, the effects of these factors can be more severe with more speed. For example, a deer running across the road has no consequences to a vehicle parked at 0 mph but could have disastrous consequences for a vehicle traveling at 100 mph. This suggests that lower speeds can reduce the frequency and severity of crashes; lower speeds can give the driver more time to respond appropriately in the face of unexpected dangers, and it can reduce the severity of a crash should one happen. However, since the efficacy of speed limits in restraining driver speed is subject to debate, it is not clear how well speed limits can ameliorate these other factors.

Another view is that, while speed can play a part of the causal chain which leads to crashes, speed's role is mostly to magnify the consequences of other unsafe acts. This viewpoint is reinforced by the fact that speed is rarely the sole crash factor. In many cases, removing the other crash factors, such as a right of way violation, would have absolutely prevented the collision. However, while reducing the speed could have a beneficial effect on the severity and probability of the crash, it usually cannot guarantee crash prevention.

Variable speed limits

Recently some jurisdictions have begun experimenting with variable speed limits which change with road congestion and other factors (this is distinct from France's reduction of limits during adverse weather). One example is on Britain's M25 motorway, which circumnavigates London. On the most heavily-traveled 22 kilometre section of the M25 variable speed limits combined with automated enforcement have been in force since 1995. Initial results of the 1995 trial indicated savings in journey times, smoother flowing traffic and a fall in the number of accidents, so the trial implementation was made permanent in 1997. Further trials on M25 have been thus far inconclusive [6].

Inconclusive results from most speed limit studies. For example, a 1972 OECD Road Research Group report entitled 'Speed Limits Outside Built-Up Areas' reviewed most international studies to that date. They concluded that "because of the weaknesses in the research designs of many investigations, scientifically well-established conclusions cannot be drawn." "Indeed, some of the speed limit changes were more in the nature of administrative exercises than scientifically designed experiments and the methods of analysis in these cases were deficient from the statistical point of view." The Group stated that "speed limit policies should be based on reliable research work and generally accepted scientific evidence". They proposed an international co-operative experiment to overcome weaknesses in prior studies. However, the 1973-1974 oil price crisis intervened, and wide-spread blanket speed limits became more common without exacting study.

Definition of 'speeding' or 'speed-related' to encompass two distantly-related concepts belies the presumed, strict relationship between speed and safety.

The 1998 U.S. Federal Synthesis found "limited evidence that suggests the net effect of {higher motorway} speed limits may be positive on a system wide basis {by shifting more traffic to these safer roads}. More research is needed to evaluate the net safety effect of speed limit changes".

Motorists generally pick reasonable speeds for conditions, even on motorways[7]. For example, the 75-mph speed limit in the U.S. State of South Dakota has good compliance: the average speed is less than or equal to the posted limit almost a decade after it was increased [8].

Prior to the (now defunct) 1974 national 55-mph speed limit in the U.S., German Autobahns had a higher fatality rate than U.S. Interstates; however, a few years later, the Autobahn rate fell below that of (then) 55-mph limited U.S. Interstates. IRTAD records show the U.S. rate remains higher than that on the largely unrestricted German Autobahn network. While the fatality rate on the UK's 70-mph speed-limited motorways is about half of Germany's, the 60-mph limit in rule-conscious Japan corresponds to a motorway fatality rate greater than Germany's. However, simple comparisons of fatality rates between countries neglect to account for differences in traffic density, quality of medical care, and Smeed's law.

Footnotes

Template:Note The usage of arbitrary here is not meant to convey a point of view. Rather, it is meant to convey that the selection of speed limits in the United States is often constricted or absolutely specified by broadly applicable legislative fiat. For example, it is no coincidence that virtually all rural speed limits match a speed that is prescribed in statutes. Commonly accepted speed zoning procedures would usually result in higher limits.